Date   : Fri, 30 Jul 2010 22:52:09 +0100
From   : afra@... (Phill Harvey-Smith)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

Steven Flintham wrote:
> On 30/07/10 21:58, Rick Murray wrote:
>> On 30/07/2010 21:44, Steven Flintham wrote:
>>> P.S. I tried to find a manual or something to confirm the existence of a
>>> DDFS offering 720KB, but surprisingly failed
>> You won't find it. Maths again. You'll need nine sectors per track for
>> eighty tracks on two sides. Oh, and 512 byte sectors. I've just
>> described a DOS disc. :-) Needs 512*9 for that to work. You'll be able
>> to write software to access these discs, but you won't find a
>> conventional (D)DFS offering them, they're all 256/sector.

I don't get what you're getting at 720K is cirtainly possible with an 8 
bit micro, Dragon Dos on the Dragon 32/64 could format either single or 
double sided 40 or 80 tracks. It used 18 x 256 byte sectors per track 
giving : ss40=180K, ss80=360K, ds40=360K, ds80=720K. This is with a 
0.98MHz processor so it isn't a processor speed restriction, as that is 
less than half the BBC's 2MHz.

Or is there something about the DFS / ADFS filesystems that would make 
this difficult ?

> I was so sure I had not made this up :-) I just went and looked in a 
> copy of The Micro User (July 1987, FWIW). I have a Watford Electronics 
> ad in front of me saying their DDFS "gives 80% more storage per disc in 
> double density mode, P.S. - Please note that not all DDFSs are capable 
> of providing the full 80% storage increase", and I make that 720K for a 
> double-sided 80 track disc (400K with standard DFS, 400*1.8=720). Am I 
> being dense (no pun intended!) or is the ad misleading?
> 
> I like your reasoning that 1024 byte sectors are bad news on a machine 
> like the BBC, except for claims like Watford's. Maybe their DDFS raised 
> PAGE higher than &1900, I have no idea...

Nope I tend to use WDDFS in my 1770 equipped BBCs and if I remember 
correctly page is not set any higher than normal, so I would assume that 
it also uses 18x256byte sectors.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.